1. Field of the Invention
The present invention relates to a method and device for the correction of sounds for hearing-impaired individuals. It can be applied equally well to the making of auditory prosthetic devices as well as to software that can be executed on personal computers or telephone answering machines and more generally to any device designed to improve hearing comfort and the understanding of speech by persons affected by deafness.
The problem of deaf people essentially arises out of the specific and degraded nature of their auditory perception.
In his need to communicate, man since the dawn of time has constructed a mode of oral communication, namely speech, based on the mean characteristics of the production of sound signals (in the form of voice) and their perception (by the ear). Everyday language therefore is the language of the greatest number. By contrast, the hearing of the hearing-impaired person is far removed from the mean and everyday language hardly or even not at all accessible to him.
The understanding of everyday language is a prerequisite for the integration of a hearing-impaired person into his community. In what may be considered to be a reflex of social survival, any hearing-impaired individual is naturally left to construct a language of his own and implement methods, techniques and a strategy of communication that enable him to transpose the common language into his own specific language. A known spectacular example is that of lip-reading which enables access to normal speech through a visual alphabet of the position of the lips.
The twentieth century has seen a constant effort in the designing of machines designed to relieve hearing-impaired individuals and help them.
2. Description of the Prior Art
Two classes of machines have been developed.
A first class of machines deals with xe2x80x9clightxe2x80x9d deafness and is aimed at correcting hearing and making it as normal as possible. This is what done by the usual prosthetic devices that are widely available in the market.
A second class of machines pertains to more extreme cases of deafness and seeks to convert speech into synthetic speech accessible to the hearing-impaired person. In this category, most of the achievements relate to xe2x80x9cheavily deaf individualsxe2x80x9d. A remarkable example is that of the cochlear implant which acts by means of electrodes applying direct stimulation to the auditory nerve.
The present invention seeks to propose a solution for persons suffering from what is known as xe2x80x9cintermediatexe2x80x9d deafness. These persons presently have no appropriate technical aids. They are far too afflicted to be helped by the usual forms of prosthesis but their auditory abilities are sufficient for them to be able to do without the devices used for people afflicted with heavy deafness.
The usual prosthetic devices generally implement a method of selective amplification of speech as a function of frequency. In its implementation, an automatic system for the regulation of the sound level acts on the amplification gain. The aim is to provide the best possible hearing comfort and protection against instantaneous power peaks.
For reasons of business strategy and in response to requests by patients, these prosthetic devices are miniaturized so that they can fit into the curve of the ear or be inserted therein, leading to relatively mediocre performance characteristics capable of providing only very approximate levels of auditory correction. Typically, only three frequency bands are defined for the frequency correction. These prosthetic devices, without doubt, deal with xe2x80x9clightxe2x80x9d deafness which is the most frequent type of deafness. Heavier deafness may be relieved but at the cost of painful disadvantages caused especially by the amplification of the background noise and by Larsen""s phenomenon. Furthermore, there is no possibility of correction in the frequency zones for which there is no hearing.
In the history of prosthetic devices for heavy deafness, reference may be made to the work by J. M. TATO, Professor of E.N.T. medicine, and Mr. VIGNERON and Mr. LAMOTTE quoted in the article by J. C. LAFON, xe2x80x9cTransposition et modulationxe2x80x9d (Transposition and modulation), Bulletin d""audiophonologie annales scientifiques de Franche Comtxc3xa9, Vol. XII, No. 3 and 4, Monograph No. 164, 1996. These prosthetic devices exploit the fact that deaf people are rarely completely deaf and that a very small residue of perception persists, often in the low-pitched tones. It has often been attempted to put these facts to profitable use.
Thus, it is possible to very approximately restore a perception of sound to deaf people by what are called methods of xe2x80x9ctranspositionxe2x80x9d from the high-pitched tones to the low-pitched tones. Unfortunately, the understanding of language requires more than a simple perception, and it turns out to be the case that the transmission of intelligibility is inseparable from a necessary xe2x80x9crichnessxe2x80x9d of the sound. Restoring this xe2x80x9crichnessxe2x80x9d has become one of the main subjects of preoccupation. Thus, the creation of a synthesized speech has been envisaged in order to restore the structural elements that form the medium for the intelligibility of everyday language.
The techniques implemented in 1952 by J. M. TATO consisted in recording speech spoken very swiftly and then restoring it at half speed. This enabled a transposition by one octave towards the low-pitched tones while preserving the structure of the initial speech. Tests have shown that this has a certain advantage for deaf people.
However, the drawback of this method is that it can be used only in deferred time. The technique developed in 1971 by Mr. VIGNERON and Mr. LAMOTTE enables a  less than  less than real-time  greater than  greater than  adaptation of this method, in which the time is cut up into intervals of {fraction (1/100)} seconds with the elimination of one in every two intervals, J. M. TATO""s method being applied to the remaining intervals. However, this system unfortunately has a high level of background noise.
The idea of building xe2x80x9cnaturalxe2x80x9d sounds is also present in a prosthetic device also known as xe2x80x9cGALAXIExe2x80x9d in the article by J. C. LAFON. This prosthetic device implements a battery of filters and mixers distributed over six subbands and achieves a transposition into the low-pitched tones used for people afflicted with heavy deafness.
Unfortunately, these methods work at the level of the signal and have far too many distortions and far too much of hearing discomfort to be used by persons suffering from intermediate deafness.
The article by Mr. Jean Claude LAFON brings out three main guidelines that may be used to obtain efficient prosthetic treatment.
1xe2x80x94It appears to be important to be able to transpose the totality of the sound structure, namely to take the structural elements of speech that carry intelligibility into the zone of perception of the hearing-impaired individual.
2xe2x80x94It appears to be also important to produce xe2x80x9cnaturalxe2x80x9d sounds, namely to reproduce synthetic speech that carries information having a structure that is in harmony with the auditory capabilities of the hearing-impaired individual.
3xe2x80x94Finally it is necessary to ensure the preservation of the temporality of the speech signal, for rhythm is a carrier of information accessible to the hearing-impaired individual.
The original idea of the invention is to overcome the above-mentioned drawbacks by using a parametrical model of the speech signal capable of making relevant conversions in order to achieve auditory correction for hearing-impaired individuals in implementing a method capable of meeting the three constraints referred to here above.
To this end, an object of the invention is a method to provide auditory correction for hearing-impaired individuals that consists in extracting the parameters characterizing the speech, the voicing, the energy and the spectrum of the speech signal, modifying the parameters to make the speech intelligible to a hearing-impaired individual and reconstructing a speech signal perceptible to the hearing-impaired individual by means of the modified parameters.
An object of the invention is also a device for the implementing of the above-mentioned method.
The method and device according to the invention have the advantage of implementing the parametrical models that are commonly used in vocoders in order to adapt them to hearing by hearing-impaired individuals. This makes it possible to work no longer at the level of the sound signal as is done in the prior art techniques but at the level of the symbolic structure of the speech signal in order to preserve its intelligibility. The vocoders indeed have the advantage of using an alphabet that incorporates the notions of xe2x80x9cpitchxe2x80x9d, xe2x80x9cspectrumxe2x80x9d, xe2x80x9cvoicingxe2x80x9d, and xe2x80x9cenergyxe2x80x9d which are very close to the physiological model of the mouth and the ear. By virtue of Shannon""s theory, the information transmitted is then truly a carrier of the intelligibility of speech. The concrete representation of the intelligibility of speech in computer form thus opens new prospects. Intelligibility may thus be acquired during the operation of analysis and is restored during the synthesis.
Through the invention, the operation of synthesis of a parametrical vocoder may thus be matched with the auditory characteristics of hearing-impaired individuals persons. This technique, associated with more conventional methods, makes it possible to envisage a particularly general method of prosthesis that can serve a very wide population, especially people suffering from intermediate deafness.
Another advantage of the method and device of the invention is that it provides great freedom in the settings, each parameter being modified independently of the others without any reciprocal impact, with a specific setting for each ear.